The present invention relates to an electronic device coated with resin and a method producing the same.
Most ceramic filters, hybrid ICs and ceramic capacitors (as well as many other electronic devices) include a sheath covering the internal electronic element to protect the device from external shock. The sheath is typically made from a thermosetting resin which is resistant to both heat and physical shock. An exemplary structure is shown in FIG. 11. The device 20 includes an electronic element body 21, lead terminals 25 which are electrically connected to the body 21, and an outer sheath 22 covering the body 21. The outer sheath 22 is formed of a thermosetting resin.
The outer sheath 22 is typically formed by dipping the body 21 into a resin paste (prepared by dispersing a coating material of a thermosetting resin in an organic solvent) to adhere the resin paste to the body 21 and then drying the paste and removing the organic solvent from the resin paste through evaporation in air at room temperature. These steps are repeated until a desired thickness of the thermosetting resin has been applied to the electronic element body 21. Next, the thus-coated electronic device 20 is heated at a predetermined temperature to cure the thermosetting resin with the result that the electronic element body 21 is protected with the cured resin.
When the electronic device 20 has vibrating components such as electrodes 24, an empty space 23 must be formed around each such vibrating element to ensure that it is free to vibrate. To this end, a thermo-subliming material (such as wax) is applied to the region corresponding to the empty spaces 23 prior to applying a thermosetting resin onto the body 21. Then, the body 21 is processed as described above. The thermo-subliming material has a sufficiently low melting point that it will melt during the heating step used to cure the thermosetting resin, will evacuate the area corresponding to the empty spaces 23 and will be absorbed in the pores of the outer sheath 22.
The conventional electronic device 20 and the method for producing it have the following drawbacks.
The porosity of the thermosetting resin used for forming the outer sheath provides a good impact resistance, makes it easy to mark the surface of the outer sheath and absorbs the wax or other thermo-subliming material which has been evaporated from the empty spaces 23. However, it exhibits a poor moisture resistance, with the result that the characteristics of the electronic devices are often degraded.
An object of the present invention is to provide an electronic device coated with an outer sheath that is resistant to both physical shocks and moisture, and to provide a method of producing the same.
The present invention includes an electronic device, comprising:
an electronic element;
an outer sheath covering at least substantially all of the outer surface of the electronic element, the outer sheath comprises at least one thermosetting resin layer and at least one thermoplastic resin layer.
The thermoplastic layer is preferably significantly less porous than the thermosetting layer after both layers have been cured with the thermosetting layer preferably having a porosity of about 25 to 40 percent. The thermoplastic layer may have a porosity of about 30 to 60 percent before curing. The thermoplastic layer is preferably a non-porous film layer after curing.
The outer sheath is preferably formed of three layers. A first thermosetting layer covers at least substantially the entire outer surface of the electronic element, a thermoplastic layer covers at least substantially the entire outer surface of the first thermosetting layer and a second thermosetting layer covers at least substantially the entire thermoplastic layer. The first and second thermosetting layers are preferably formed of the same material. When the electronic element includes vibrating portions, empty spaces are formed by areas of the outer sheath which are spaced from the outer surface of the electronic element body.
The electronic device is formed by a method comprising the steps of:
(1) forming a green outer sheath on an outer surface of an electronic element, the outer sheath including at least one thermosetting resin layer and at least one thermoplastic layer;
(2) heating the green outer sheath to thermally cure the thermosetting resin layer and melt the thermoplastic resin layer; and
(3) solidifying the melted thermoplastic layer.
In the preferred embodiment, the green outer sheath comprises a first thermosetting resin layer covering at least substantially the entire outer surface of the electronic element, a thermoplastic layer covering at least substantially the entire outer surface of the first thermoplastic layer and a second thermosetting resin layer covering at least substantially the entire outer surface of the thermoplastic resin layer and wherein both the first and second thermosetting resin layers are cured during the heating step. When it is desirable to form empty spaces in the outer sheath, for example when the electronic element includes vibrating portions, the method further comprises the step of placing a thermo-subliming material, for example wax, on a portion of the outer surface of the electronic element before the step of forming the green outer sheath so that the green outer sheath is spaced from associated portions of the outer surface of the electronic element. In this case, the heating step is carried out in such a manner that the thermo-subliming material melts and dispersed into the first and second thermosetting resin layers before the thermoplastic resin layer is melted and empty spaces are left in the area where the thermo-subliming material was originally placed.
The heating step is carried out by first heating the green outer sheath at a temperature not lower than the melting point of the thermo-subliming material but lower than the melting point of the thermoplastic resin layer to cause the thermo-subliming material to evacuate the empty space and disperse into the first and second thermosetting resin layers and then heating the outer sheath at a temperature not lower than the melting point of the thermosetting resin layer to thermally cure the first and second thermosetting resin layers while melting the thermoplastic resin layer.
The green sheath is preferably formed by:
(1) applying a thermosetting resin to at least substantially the outer surface of the electronic element to form the first thermosetting resin layer;
(2) applying a thermoplastic resin to at least substantially the first thermosetting resin layer to form the thermoplastic resin layer; and
(3) applying a thermosetting resin to at least substantially the outer surface of the thermoplastic resin layer to form the second thermosetting layer.
According to the preferred embodiments of the invention which includes a three layer outer sheath, the outer sheath is advantageous in that the first and second thermosetting resin layers absorb external shock while preventing it from being transmitted to the electronic element, and that the thermoplastic resin layer protects the electronic element from moisture in air. Accordingly, the electronic device of the invention has good moisture resistance, and the adhesion between the electronic element and the outer sheath in the device is good. In addition, since the second thermosetting resin layer is the outermost layer of the outer sheath, the outer sheath is resistant to external shock and is easy to mark.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.